The retroviral Human Immunodeficiency Viruses 1 and 2 (HIV) are the most common causative agents of AIDS. Through a portion of a viral envelope protein (gp120), HIV binds specifically and with high affinity to the CD4 molecule on T-lymphocytes. Following binding, the virus fuses with the cell membrane and is internalized. Within the cell, it produces a reverse transcriptase which transcribes its genomic RNA to DNA. The reverse HIV transcript is then integrated into the cellular DNA where it exists for the life of the cell as a “provirus.” The provirus can remain latent for an indefinite period of time, or it can be “activated” to transcribe mRNA and genomic RNA, leading to protein synthesis, assembly, new virion formation, budding of virus from the cell surface, and cell death.
While the precise events triggering activation are poorly understood, they appear to lead to liberation/production of endogenous cellular factors that interact with the HIV genome to promote translation. In this regard, binding of cellular SP1 to the HIV promoter (which contains several tandem SP1 consensus binding sites) is needed for high-level transcription of the latent HIV genome. Additionally, NFκB functions as a potent transcriptional activator when it binds to one or two (depending on the HIV strain) consensus binding sites in the HIV enhancer, which is adjacent to the promoter. The transcription factors CREB/ATF, NF-AT, and AP1 also potentiate HIV transcription. As for all retroviruses, the structural and enzymatic gag, pol and env gene products are produced when the provirus is activated. HIV first transcribes gag-pol as a fusion protein which is ultimately cleaved by the HIV protease enzyme to yield the mature viral proteins. HIV also employs additional regulatory proteins (specifically the tat and rev gene products) as transcriptional enhancers to induce high levels of gene expression. Nef is another HIV gene that modulates viral replication levels.
While the set of factors triggering active viral replication remains only partially understood, some of them include heat shock, ultraviolet radiation, regulatory proteins of other (e.g., superinfecting) viruses, inflammatory cytokines (e.g., IL1, IL2, IL4, IL6, IL10, Tumor Necrosis Factor α (TNFα), Platelet Activating Factor, Interferon γ (IFNγ)), and Nitric Oxide. Many of these factors are T-cell activators (e.g., they precipitate cell cycling and clonal expansion of T-cell populations), and they are released by many B-cells in direct response to infectious agents (such as HIV). Such factors also trigger intracellular signaling events promoting the production of NFκB and its dissociation from its inhibitor (IkB). Active NFκB is a DNA binding protein activating the transcription of many cellular genes, and also the HIV genome. In this regard, cytokines such as TNFα and IL-1 augment NFκB activity in cultured T-cells.
Some cells harboring the provirus express HIV gp41, gp120, and possibly other viral proteins, presumably through basal levels of transcription from the proviral genome. While a host immune response is mounted against such HIV proteins, due in part to the high degree of mutability of such proteins and their varied glycosylation patterns, such immune response usually is incomplete, resulting in a pool of latent virus that effectively avoids immune surveillance. Additionally, the presence of HIV gp120 in the membranes of infected cells can mediate fusion events between infected cells and non-infected antigen-presenting cells (e.g., dendritic cells) via a reaction similar to that by which the virus enters uninfected cells. Rather than destroy infected cells, as might be expected for a cellular immune response, such fusion events typically lead to the formation of short-lived multinucleated syncytial “giant cells,” which actually facilitate viral replication. In this regard, while latently-infected monocytes and T-lymphocytes normally are quiescent and have no active NFκB, other cells (e.g., dendritic cells) normally contain high levels of active NFκB. However, dendritic cells do not produce SP1, while T-cells and monocytes express Sp1 in active form. Formation of syncytia between infected T-cells and dendritic cells, thus, brings active-NFκB and SP 1 into the same cell, facilitating transcription of the HIV genome.
The viral life cycle ends when mature HIV are “budded” from the host cell, retaining some amount of cell membrane as part of its envelope. Oftentimes, these budding events are localized to areas of the cell membrane where intracellular adhesion molecules (ICAMs) and other surface receptors coalesce during the cell's activation process. Because such proteins localize to regions of intercellular contact, this phenomenon (known as polar capping or polarization) can help spread the viral infection by “focusing” viral budding to an adjacent cell or in facilitating syncytia formation. Moreover, liberated virions often contain some membrane-bound ICAMs (e.g., ICAM-1), and such viruses can bind cells (e.g., peripheral blood mononuclear cells) through interactions not involving the gp120-CD4 interaction. Such ICAM-1+ HIV viruses are more infective than ICAM-1− HIV, and since they are cloaked with the host animal's glycoproteins, they are much less likely to be neutralized by circulating host antibodies. HIV can augment production of cell adhesion molecules such as ICAM-1 by precipitating the phosphorylation of STAT1α, which binds to the ICAM-1 gene enhancer and promotes SP1-dependent transcription. Interestingly, inflammatory cytokines (e.g., IFNγ) also precipitate phosphorylation of STAT1α, and the gene contains consensus binding cites for some of the same transcription factors involved in HIV replication, notably NFκB.
The presence of latent pools of HIV within quiescent cells, the high mutability of HIV proteins and their relative invisibility to immune surveillance, and the ability of the virus to alter its tropism by acquiring ICAMs all permit the virus to replicate in the face of an aggressive host immune response. Over time, the virus gradually subverts and progressively destroys the very system relied on to ward off infections. This progression of viral persistence and replication is HIV disease, and it is marked by dysregulation of cytokine signaling, particularly in the lymphatic system, and ultimate destruction of lymph nodes. When HIV disease has progressed to the point where the host's immune system becomes so incapacitated that it is unable to ward off opportunistic diseases (e.g., bacteria, fungi, neoplasms, etc.), AIDS develops. Many patients begin to develop AIDS symptoms when their CD4+ T-cell count drops to about 200 (most healthy adults have a CD4+ T-cell count of about 1000.
To prevent the development of AIDS, many current therapies focus on halting viral life cycle events, typically by directly targeting viral proteins. For example, gp120 antibodies have been produced in an attempt to block initial cell infection. However, due in part to the ability of the virus to spread by syncytia or direct cell-to-cell contact and its ability to acquire ICAM molecules, such attempts have met with mixed results. Other therapies employ inhibitors of HIV protease to block the formation of mature rep and cap from the rep-cap preprotein. Still other regimens employ combinations of antiviral compounds, aimed at inhibiting or attenuating viral enzymes. It has been estimated, however, that spontaneous mutations arise in HIV genes once in about 104 replications (Perelson et al., Science, 271, 1582-86 (1996)). Given that the virus typically undergoes about 1010 replications each day, resistance to agents acting directly against viral proteins is not uncommon. Moreover, many regimens require a patient to adhere to very a strict dosing schedule involving scores of pills each day. Failure of patients to comply with such regimens adds to the failure rate of antiviral therapy. In light of these problems, there is a need for new methods, compounds, and compositions for attenuating the progression of HIV disease and other immune dysfunctions.
Many thousands of people are diagnosed with cancer and other neoplastic disorders each year, and although advances have been made in cancer therapy, the existing treatments are not successful in many cases. For example, many anticancer drugs administered to patients often have toxic effects on non-cancerous cells in the patient's body. Moreover, many neoplastic cells whose growth can be inhibited by certain drugs sometimes become resistant to those drugs. Of course, responsive tumors represent only a small fraction of the various types of neoplastic disease and, notably, there are relatively few drugs highly active against solid tumors such as ovarian cancer, breast cancer, lung cancer and the like. Thus, patients with many types of malignancies remain at significant risk for relapse and mortality. As such, there exists a continuing need for agents that inhibit neoplastic growth, especially solid tumor growth.